I returned later to the range with three brand new buff-colored USPSA Milpark targets. When running both guns twice more through the IHS drills, the dot sights held their own against the post/notch. Frankly I could stand to see XS offer its sights with a different color than white as a option.

Here's an overview of how I performed with the sights on the various drills.

Drill 1 Double-tap one target. Average shot-to-shot speed: WTS .24 second. XS .24 second. Not a whole lotta difference there.
Drill 2 One shot on each of two targets. WTS .37 second. XS .38 second.
Drill 3 Fire one shot, speedload, fire one more shot on the same target. This is actually an excellent test of sight visibility since you will lose the sights during the speedload, then reacquire them before firing the next shot. WTS 1.66 seconds. XS 1.82 seconds. A bit more difference here, though I'll be the first to admit it's debatable how important this is anywhere outside a combat pistol match.
Drill 4 Ready position. I prefer the Low Ready, with gun aimed at the lower edge of the target as if covering the opponent's pelvis while still leaving his hands visible. At the timer's signal, raise the gun and fire one round to the chest. WTS .40 second. XS .42 second.
Drill 5 Draw and fire one shot on one target. WTS 1.26 seconds. XS 1.24 seconds.
Drill 6 Draw and hit three targets once. WTS 2.23 seconds. XS 2.21 seconds.

At distance the post/notch sights did seem to offer more precision than the XS dot sights, at least in my hands. In fast and close shooting, differences in performance pretty much went away (with the exception of the speedload, which could be easily chalked up to me rather than the sights), with best speed sometimes coming from the post/notch, sometimes the dot, and could be measured in mere hundredths of a second either way.

While I have no plans to knock the WTS sights off my own Glock 17, the results of this test have left me convinced that I would never say a shooter who prefers the big dot system is "wrong." Both types of sights can work, and work well.